Acid-Base Reactions Affecting Half-Wave Potentials

Acid-Base Reactions Affecting Half-Wave Potentials 

For systems which involve rapidly established acid-base equilibria preceding the electrode process proper, the polarographic curves show a single wave the height of which is pH-independent. For the simple scheme (26)  

This equation indicates that for [H+] K, Ej = Constant and is pH-independent, whereas for [H+] K, Et = Constant — (2.3 RTjanF) pH. The pH-dependence of the half-wave potential thus consists of two linear parts at pH (pK — 1), the half-wave potential is pH-independ-ent at pH (pK + 1), it is shifted to negative potentials with a slope 0.059/a . The intersection of the two linear parts corresponds to pK. 

There are two factors which limit the application of this treatment. For many of the compounds studied (e.g. carbonyl compounds, nitrocompounds etc.), the conjugate acid is so strong that the pK value lies beyond the accessible acidity range. The other limitation is inherent in the assumption that the electrode process (26 b) does not involve any further proton transfer. For numerous systems, in addition to a rapidly established protonation in an acidic grouping placed at some distance from the electroactive group, a further proton transfer to the electroactive group itself occurs at the electrode surface. It is therefore desirable 

These conditions are fulfilled for the reduction of phenacylsulpho-nium salts at low pH values cleavage of the C—S bond occurs in the first step and proton transfer is involved only in the consecutive steps. The observed shift of half-wave potentials (74) with pH follows the plot predicted by equation (27). The intersection of the two linear parts indicates that phenacylsulphonium salts are moderately strong acids with pK values between 7 and 8 (Fig. 20). Potentiometrically measured pK values, used for verification, are in good agreement with the approximate pK values obtained from polarographic data. The system involved in the first step can be described by scheme (28)  

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